1. Field of The Invention
This invention relates to a cordless telephone which selects and uses one of radio channels.
2. Description of The Related Art
A conventional cordless telephone uses one of several channels, which is selected by a user thereof, to perform communication between a base unit (hereunder often referred to as a master unit) and a portable unit (hereunder frequently referred to as a slave unit) thereof. Such a conventional cordless telephone is provided with a backup circuit in the master unit, powered by a rechargeable battery for forming a backup of information transmitted through the currently used channel to prevent the information from being lost when a power line is disconnected from the master unit. Hereinafter, the conventional cordless telephone will be described by referring to the accompanying drawings.
FIG. 5 is a schematic block diagram for illustrating the configuration of a conventional cordless telephone. Reference numeral 1 designates a master unit connected to a telephone: line; 2 a receiving portion; 3 a transmitting portion; 4 a controlling means; 5 a channel storing portion; and 6 a channel backup circuit.
Further, reference numeral 7 denotes a slave unit; 8 a receiving portion; 9 a transmitting portion; 10 a control means; and 11 a channel storing portion. Further, reference numerals 100 and 200 designate transmission/reception antennas. Incidentally, it is assumed that a radio channel used in this conventional cordless telephone is a specific channel (hereunder referred to as an N-channel or as a channel N).
Even when a Dower lane is disconnected from the master unit during communication between the master and slave units, both of which use the N-channel, the identification of the N-channel is maintained as a channel to be used by the master unit under the action of the channel backup circuit 6 since the identification of the last channel used is stored.
Thereafter, the power line is connected to the master unit again. Subsequently, an operation of transmitting data from the slave unit to the master unit is performed as follows. Namely, the control means 10 of the slave unit 7 transmits to the master unit 1 channel data indicating the N-channel, which is currently used by the slave unit 7, and identification code indicating that the slave unit 7 and the master unit 1 are coupled with each other. The channel data and the identification code are collectively called slave-unit confirming data. Namely, the slave-unit confirming data are transmitted from the slave unit 7 to the master unit 1 in order to confirm that the communication channel used by the slave unit 7 is the same as that used by the master unit 1. When power is lost by the master, such confirmation does not occur until power reconnection.
If the receiving channel used by the master unit 1 is the same as the transmitting channel used by the slave unit 7, the master unit 1 can receive the slave-unit confirming data. When the master unit 1 receives the slave-unit confirming data, the master unit 1 checks whether or not the identification code of the received slave-unit confirming data indicates that the same channel is used by the master and slave units (namely, the slave unit having transmitted the slave-unit confirming data and the master unit are coupled with each other). If it is confirmed that the same channel is used by the master and slave units, the master unit 1 replies by sending the channel data and the identification code to the slave unit 7 as master-unit confirming data. Thus a call can be made (namely, voice communications can be established) therebetween.
Such a conventional cordless telephone, however, has the following drawbacks. Namely, even when the master unit thereof is disconnected from an external power source, the master unit thereof should store information on a channel to be used fop communication between the master unit and a slave unit thereof. Thus the master unit needs a backup circuit. Consequently, the conventional cordless telephone becomes expensive. In addition, the performance of the backup circuit limits a period of time in which information representing a currently used channel is stored therein in case where a power source is disconnected from the master unit. Thus, if the power source is disconnected from the master unit for a long time, the information representing the currently used channel is sometimes lost. This results in that no call can be made between the master and slave units thereafter and the master and slave units cannot revert to a state in which a call can be made therebetween. The present invention is accomplished to eliminate the drawbacks of the conventional cordless telephone.